Recently, in applications such as label package doubling as a protection of a glass bottle and a PET bottle etc. and display of articles, cap sealing and accumulation package, there have been widely used drawn films (so-called heat-shrinkable films) composed of a polyvinyl chloride resin, a polystyrene resin, a polyethylene resin or the like. Of these heat-shrinkable films, a polyvinyl chloride film has problems that heat resistance is low, and it generates hydrogen chloride gas in incineration and causes dioxin. A polystyrene film has problems that it is inferior in chemical resistance, as well as an ink with a special composition needs to be used in printing, it requires high temperature incineration and generates a lot of black smoke accompanied by an abnormal odor. Therefore, as a shrink label, there has been widely used a polyester heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in solvent resistance, and the use amount tends to increase being accompanied by an increase in turn volume of PET containers.
As a heat-shrinkable polyester film, one which is allowed to shrink greatly in the width direction has been widely utilized up to the present time. Although the heat-shrinkable polyester film in which the width direction is the main shrinking direction is subjected to drawing at a high ratio in the width direction in order to allow the shrinkage properties in the width direction to be developed, with regard to the longitudinal direction orthogonal to the main shrinking direction, there have been many cases in which the film is only subjected to drawing at a low ratio and there is also a case in which the film is not subjected to drawing. The film subjected to drawing at a low ratio in the longitudinal direction and the film subjected to drawing only in the width direction have a drawback that the mechanical strength in the longitudinal direction is poor.
Moreover, in the case of preparing a label for a bottle for beverage from a heat-shrinkable film, it is necessary that the film be formed into a tube shape to be mounted to a bottle and then be allowed to shrink by heat in the peripheral direction of the bottle. On that account, in the case of forming a heat-shrinkable film which is heat-shrinkable in the width direction into a label, while a tube body is formed so as to allow the width direction of the film to become the peripheral direction, this tube body must be cut into a label with a prescribed length to be mounted to a bottle. Accordingly, there has been a restriction on the speed for mounting a label composed of a heat-shrinkable film which is heat-shrinkable in the width direction to a bottle, and improvement thereof has been desired.
On that account, recently, a film which is capable of being wound around a bottle directly from a film roll to be formed into a label (the so-called wrap-around) and is heat-shrinkable in the longitudinal direction has been desired. Furthermore, in recent years, a wrapping method of covering the circumference of a synthetic resin-made container for a box lunch or the like with a belt-shaped film to hold the container in a closed state has been developed, and the film which is shrinkable in the longitudinal direction is suitable for such a packaging application. Accordingly, the demand for the film which is shrinkable in the longitudinal direction is expected to dramatically increase in the future.
It is known for a long time that, at the time of producing a label from a film which is shrinkable in the width direction, bonding is performed using an organic solvent (Patent Document 1 and the like). In contrast, with regard to a film which is heat-shrinkable in the longitudinal direction, a method of allowing an end part of the film to adhere by means of heat sealing, a hot-melt adhesive, or the like while allowing the film to be wound around a bottle has been employed. However, when the speed of a label mounting line is highly quickened for enhancing the productivity, sufficient adhesive strength is not attained, and a problem that being turned up or peeling occurs in a bonded part at the time of heat shrinkage has been generated.
Furthermore, with regard to films which are shrinkable in the longitudinal direction (Patent Documents 2 to 4), when the heat shrinkage in the longitudinal direction is attempted to be heightened, the maximum shrinkage stress is also heightened and being turned up or peeling occurs in a bonded part of a label at the time of shrinkage. That is, these problems have been generated because the heat shrinkage in the longitudinal direction and the maximum shrinkage stress correlate with each other.
On the other hand, the present applicant has consistently continued the development of a heat-shrinkable polyester film and has published a number of techniques. However, as the amount of consumption of PET bottles has been expanding in recent years, the production at a faster rate has been desired, and a heat-shrinkable polyester film more excellent in shrinkage finishing properties has been desired.